Good Question – Washboard Effect On Dirt Roads

By: Jeff Herndon Email
By: Jeff Herndon Email

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Thursday, March 26, 2009

Mary from Norwich wants to know, “What causes washboard roads?” Good question.

The washboard effect on dirt roads is like a lot of little speed bumps. If you travel dirt roads, you know how noisy and bumpy it can be. You might be surprised at what causes it.

Mary says her friend believes that washboard roads are caused by the speed and blade settings of road graders, causing the blade to skip.

Good theory, but not correct according to and Tom Pettigrew, a forest service engineer. He says the culprit is…your car. More specifically, your car’s suspension.

Pettigrew says the suspension system distributes shock and energy on road irregularities with a bouncing rhythm called harmonic oscillation. It’s this pattern that causes the ruts. Once the ruts establish, they get worse as each car passes. Pettigrew says washboarding is inevitable on any unpaved road that sees fairly heavy traffic.

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